The exhaust gas ducting devices in accordance with the invention are e.g. mufflers, but in particular exhaust gas cleaning devices such as catalysts and particle filters.
Such can include devices inserts which are very sensitive to radial pressure. So far, these are mainly axially traversed ceramic substrates, which are wrapped with an elastic compensating element (for example in the form of a mat). If possible, these inserts are held in the outer housing in an axial and radial direction only by radial clamping. On the one hand, the clamping force must be so great that in driving operation no axial relative displacement is obtained between insert and outer housing due to the gas pressure or due to vibrations. On the other hand, the radial pressure should of course not be so great that it leads to a destruction of the insert, in particular a destruction of the pressure-sensitive catalyst or particle filter substrate. Attempts are being made now to use inserts of low weight, which heat up faster in driving operation. Such substrates for example comprise a corrugated-board-like supporting structure which is coated with catalyst material.
Mounting and clamping the insert in the outer housing is typically effected either by wrapping a sheet metal jacket around the insert, by pushing the insert into a tube, which depending on the method can be pre- and/or post-calibrated, or by closing shells. When the force applied is too great, destruction of the insert, i.e. of the substrate in the case of catalysts or particle filters, can occur.
When manufacturing exhaust gas cleaning devices a great difficulty consists in that between the substrate and the outer housing the elastic compensating element, typically the bearing mat, is provided, which ensures a pressure compensation and a constant pretension. The disadvantage of this bearing mat, however, consists in that after being compressed it is subjected to a certain settling process, referred to as relaxation, so that the pressure passed on by the same to the substrate decreases. Rebound of the outer housing after mounting and clamping likewise leads to the fact that the pressure initially applied onto the substrate, and hence the clamping force applied, decreases. Furthermore, the holding pressure of the bearing mat decreases in operation (for example due to ageing). This leads to the fact that with regard to the future safe clamping of the substrate in the outer housing even more initial pressure is exerted by the outer housing onto the insert by way of precaution and individual substrates approach the limits of stability.
To ensure a sufficiently safe clamping in the outer housing and minimum scrap rates even in the case of very pressure-sensitive inserts, it is proposed in DE 10 2006 015 657 A1 to individually load small partial areas of each compensating element in a predetermined way and plot an individual deformation-pressure curve. From this curve, a setpoint deformation of the compensating element, which is necessary to achieve a setpoint pressure, is determined. In contrast to conventional methods, the individual deformation behavior of the respective compensating element thus is considered when dimensioning the outer housing, in order to obtain a desired clamping force of the insert in the outer housing as exactly as possible.
In DE 10 2006 015 657 A1 it is explicitly stated that rather small partial areas of the bearing mat (up to a maximum of 25% of the total surface) are loaded for plotting the deformation-pressure curve, in order to keep “damages” of the bearing mat, such as the breaking or alignment of fibers, as low as possible.
It was discovered, however, that these small partial areas are not always representative for the deformation behavior of the entire compensating element, which can lead to inaccuracies when determining the setpoint deformation, and correspondingly to undesirably large deviations from the specified setpoint pressure.
Moreover, it was found that loading small partial areas creates high requirements as to the test set-up and requires an extremely exact execution of the test, in order to achieve satisfactory results. However, since the deformation-pressure curve is plotted for each individual compensating element in a mass production of catalysts or particle filters, such effort is problematic for economic reasons.
There is a need to eliminate the described problems and provide a method for manufacturing exhaust gas ducting devices, in which a rather constant, specified clamping force between insert and outer housing is achieved with little effort.